A muffler shell as contemplated is conventionally constructed of rectangular metal sheet which is rolled into a tubular form to bring two of its ends into mutually confronting approximation with each other. The borders of the confronting ends are folded in oppositely facing directions to provide complementary mateable hook elements which are interlocked with each other, rolled or otherwise compressed to provide a fairly rigid seam in a well known manner illustrated and described for example in JONES--U.S. Pat. No. 2,845,889.
Such seam is relatively thick, comprising four thicknesses of the metal sheet. The rolling process may be such as to form the seam generally on the exterior surface of the shell or the interior thereof, the former being here preferred as it leaves the shell interior relatively smooth for the accomodation of the sound attenuating elements to be subsequently installed therein.
It is known to form muffler shells from two plies of metal. The plies may be formed simultaneously and the ends hooked together in one seam. It is also known to form the plies separately and to join the ends in separate seams; such method is more time consuming, and has other disadvantages. For example, the seam of the inner ply tends to interfere with the fitting of the sound attenuating components where such seam locates internally. Where the seam locates on the exterior surface of the tube formed by the inner ply, this interferes with the fitting of the exterior ply unless special provision is made therefore.
As is well known, it is important that the seams as aforesaid be gas tight to prevent leakage of exhaust gases therethrough. Unfortunately, however, explosions--e.g. back-fires--occur within the muffler which tend progressively to spread such seams permitting gas flow therethrough and resulting, finally, in the total destruction or "blow out" of the mufflers.
Resort has been had to numerous expedients for combating this problem, but these have not proven entirely practical whether because of expense, ineffectiveness, complexity, or, in some instances, because they tend to detract from the essential sturdiness of the muffler shell.
The following U.S. prior art was developed in a patentability search of the invention, namely:
______________________________________ 85,742 1,081,348 2,187,431 2,333,887 2,845,889 3,064,336 ______________________________________